1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
The current mainstream semiconductor exposure apparatus is an exposure apparatus of the step & repeat scheme called a stepper. The stepper reduces, at a predetermined ratio, the information of light having passed through an original on which a pattern is drawn, and exposes a photosensitive agent applied on a substrate to light to transfer the pattern of the original onto the substrate, while the substrate is positioned to stand still at a predetermined position. The stepper repeats this series of exposure operations over the entire substrate surface by driving a substrate stage which holds the substrate step by step. A substrate, an original, and a photosensitive agent are commonly referred to as a wafer, a reticle, and a resist, respectively.
In contrast to a stepper which performs full-plate exposure in shots on the wafer while the wafer stands still, an exposure apparatus of the step & scan scheme called a scanner exposes a wider region while synchronously scanning the wafer and the reticle.
In general, it is necessary to perform the above-described series of exposure operations a number of times for the same wafer in manufacturing a semiconductor using an exposure apparatus. In other words, it is necessary to transfer by exposure a subsequent pattern to a shot, in which a previous pattern has already been formed, so that these patterns are accurately overlaid on each other. To meet this need, an array of shots must be measured in advance. A measurement mark can be inserted in each pattern in advance and measured using a scope. It is a common practice to use an off-axis scope (to be abbreviated as an OAS hereinafter) using non-exposure light for this measurement. Since an OAS uses non-exposure light, it can perform the measurement without exposing the resist to light, but must be arranged to be spaced apart from the projection optical system to prevent the light from entering it (see FIG. 6).
The scanner generally performs real-time focusing, i.e., performs focusing while driving the stage in exposing the wafer under the projection optical system. Focus sensors 6A and 6C are arranged in front and in the rear of an actual exposure position (slit), i.e., positions shifted from this exposure position in the Y direction, as shown in FIG. 6. When the wafer is exposed by scanning the stage from the lower side of the paper surface of FIG. 6, the Z position of the wafer surface is measured by the focus sensor 6C before exposure, and the stage is driven in the Z direction before the exposure position reaches the slit position. When the stage is scanned from the upper side of the paper surface of FIG. 6, the focus sensor 6A is used as in the focus sensor 6C. A focus sensor 6B is arranged at the slit position as well in order to confirm whether the wafer surface is focused on the lens image plane of the projection optical system. The tilt of the wafer surface in the slit can also be detected by juxtaposing a plurality of sensors in the X direction. In this manner, the focus is measured immediately before exposure. This obviates the need to measure the focus over the entire wafer surface, sustaining a high throughput.
An exposure sequence using an OAS is as follows. As a wafer is transported to the exposure apparatus, a given mark in each sample shot on the wafer is measured using the OAS. A positioning error (X, Y, and rotational components) of the wafer and an error attributed to thermal expansion of the wafer are calculated based on the measurement results. The wafer is moved under the lenses of the projection optical system, and each shot is exposed. Increasing the number of sample shots improves an overlay accuracy but decreases the throughput. Furthermore, forming a plurality of measurement marks in one shot and measuring them allows exposure compatible with the shot shape and further improves an overlay accuracy, but again decreases the throughput.
To improve both the throughput and the accuracy by simultaneously performing the measurement and exposure of each sample shot, there is a scheme which uses two wafer stages so that measurement is performed by an OAS on one stage while exposure is performed on the other stage (see FIG. 7). This scheme is commonly referred to as the twin-stage configuration, whereas a scheme which uses only one stage is commonly referred to as the single-stage configuration.
In the twin-stage configuration, as a wafer is loaded onto one wafer stage, X and Y displacements of a shot are measured by an OAS first. Next, focus sensors measure the entire wafer surface. At this time, the other wafer stage is present under the lenses of the projection optical system. The former wafer stage is moved under the lenses of the projection optical system while holding the measured wafer, and exposure is started on this stage. At this time, the latter wafer stage is driven to the OAS position, and wafer loading, OAS measurement, and focus measurement are performed on this stage parallel to exposure in the same way. As long as the OAS measurement and the focus measurement are complete until the exposure, the throughput never lowers even if the number of sample shots is increased. In this case, it is possible to simultaneously optimize the throughput and the accuracy.
Focus sensors are typically juxtaposed in the X direction. This is to shorten the measurement time by a width as wide as possible upon scanning the stage in the Y direction. In addition, laser interferometers (to be described later) are set respectively at the lens position and the OAS position.
The wafer stage is required to be driven to arbitrary positions in a two-dimensional plane (the X-Y plane) over a wide range with high accuracy. One reason is that along with advances in micropatterning of semiconductor circuits, the required accuracy is increasingly becoming stricter. Another reason is that the wafer stage must be driven over a very wide range in practice to handle situations such as an increase in the wafer size, when the wafer is driven to its exchange position, and when the marks formed on the wafer by exposure are measured at positions other than the exposure position.
A laser interferometer is commonly used to detect the position of the wafer stage. The position of the wafer stage in the X-Y plane can be measured by arranging such a laser interferometer in the X-Y plane. For example, a plane mirror 2A (to be referred to as a bar mirror hereinafter) for X-axis measurement can be mounted on a wafer stage 1 in the Y-axis direction, as shown in FIG. 1. A laser interferometer 3A-1 which measures the position of the wafer stage in the X-axis direction irradiates the bar mirror 2A with a laser beam nearly parallel to the X-axis to make reference light and the light reflected by the bar mirror 2A interfere with each other, thereby detecting the relative driving amount of the wafer stage. The same applies to measurement of the position of the wafer stage in the Y-axis direction. A rotation angle θz of the wafer stage about the Z-axis can also be detected by providing two interferometers for one or both of the X-axis and Y-axis measurements.
The wafer stage can be driven to a predetermined position by arranging an actuator (not shown) such as a linear motor in the X-Y plane based on the position information obtained by the laser interferometers.
As the NAs of lenses increase to keep up with advances in micropatterning of circuits, the error tolerance of focusing for transferring a reticle image onto a wafer narrows (the depth of focus decreases), so the required accuracy of positioning in the focus direction (Z direction) is becoming stricter. For this reason, the position of the stage in the Z direction (focus direction) perpendicular to the X-Y plane, and the tilts of the stage in the X- and Y-axis directions must also be measured and controlled with high accuracy. The tilt in the X-axis direction is a rotational component about the Y-axis, and is commonly referred to as θy. The tilt in the Y-axis direction is a rotational component about the X-axis and is commonly referred to as θx. Under the circumstances, a tilt measurement scheme has been proposed. In this scheme, two X-axis interferometers 3A-1 and 3A-2 are juxtaposed in the Z direction, and position measurements are performed by simultaneously using them, thereby measuring a tilt θy of the stage in the X direction from the difference between the obtained measurement data. Likewise, a tilt θx in the Y-axis direction can be measured by juxtaposing two Y-axis interferometers 3B-1 and 3B-2 in the Z direction (see FIG. 1).
A method of measuring the position of the wafer stage in the Z direction using a laser interferometer has also been proposed. FIG. 2 shows an example of the configuration of a Z laser interferometer for detecting the position of the wafer stage in the Z direction. Laser light is perpendicularly reflected upward by a reflecting mirror 4A mounted on the stage. A 45° reflecting mirror 4B is mounted on a lens base serving as a reference, and horizontally reflects the laser light. A 45° reflecting mirror 4C is set near the lens center of the projection optical system, and projects the laser light perpendicularly downward. A reflecting mirror 4D is arranged on the stage, and perpendicularly reflects the laser light which then traces back the way it came. A reflecting mirror 4A is provided on the stage, and laser light reflected by it moves in the X direction as the stage X position moves. Accordingly, bar mirrors extending in the X direction are prepared as 45° reflecting mirrors 4B and 4C. This makes it possible to always apply laser light at the same position on the stage even when the stage moves in the X direction. When the stage moves in the Y direction, the positions of both the reflecting mirror 4A and laser light reflected by it remain the same because the reflecting mirror 4A is mounted on the X stage. At this time, since the position of the laser light relative to the Y position of the stage moves, a bar mirror 4D extending in the Y direction is provided on the stage. This makes it possible to always apply laser light onto the mirror surface on the stage even when the stage moves in the Y direction. This, in turn, always allows measurement by a laser interferometer even when the stage moves in the X-Y plane.
The relative position between the stage and the lens base surface can also be measured by mounting a 45° bar mirror 4E extending in the Y direction on the stage, and installing a bar mirror 4F extending in the X direction on the lens base, in addition to the measurement equipments described above (see FIG. 3).
In both the configurations shown in FIGS. 2 and 3, similar configurations are applied at the left and right sides (to be referred to as the L and R sides, respectively, hereinafter). As long as the measurement equipments on the L and R sides can simultaneously measure the Z positions of the stage over the entire X-Y plane, it is possible to more precisely measure the final Z position of the stage using the average of the obtained measurement results. It is also possible to measure the tilt of the stage by measuring the difference between these measurement results.
When the stage is driven in the X direction, the stage positioning accuracy in the X direction is influenced by the flatnesses of the bar mirrors 4B, 4C, and 4F extending in the X direction. Likewise, when the stage is driven in the Y direction, the stage positioning accuracy in the Y direction is influenced by the flatnesses of the bar mirrors 4D and 4E extending in the Y direction. Although a positioning accuracy in the Z direction on the nanometer order is required as described above, it is technically difficult to process the entire surfaces of bar mirrors with an accuracy on the nanometer order and assemble the thus processed mirrors.
Japanese Patent Laid-Open No. 2001-015422 proposes a technique of improving the positioning accuracy in the Z direction by measuring in advance a Z error attributed to stage driving in the X and Y directions using focus sensors built in the apparatus and determining the target position of the stage by taking account of the measurement result. A Z error attributed to stage driving in the X and Y directions is an error attributed to the processing accuracy of the bar mirrors, and will be referred to as an error of the moving plane of the stage in the following description.
In this technique, the surface of a wafer mounted on the stage or a reflective flat surface in place of a wafer is measured using focus sensors. At this time, the measurement accuracy is influenced by the wafer surface shape under normal conditions. In this technique, however, a plurality of focus sensors are used to eliminate the influence of the wafer surface shape, and measure an error in the moving plane of the stage alone. Referring to FIG. 5, a certain measurement point P on the wafer is measured by a focus sensor 6A, and is then measured by another focus sensor 6B by driving the stage. Regardless of the wafer shape, the focus sensors 6A and 6B are expected to output the same measurement value because they measure the same measurement point P. In fact, different measurement values are obtained by these sensors because an error in the Z direction is included in the measurement values due to stage driving, i.e., the measurement values are influenced by the processing accuracy of the bar mirrors. Hence, the shapes of the bar mirrors can be measured free from the influence of the wafer surface shape by the technique described above.
The thus obtained shape of the Z bar mirror is stored in a memory of a stage control processor (not shown). When the stage is to be driven in an actual exposure sequence, the position of the Z bar mirror in the Z direction can be corrected by calculating a correction value for the Z bar mirror from the target position of the stage. This makes it possible to position the stage at an ideal position at which an error in the shape of the Z bar mirror is corrected.
The measurement of a Z bar mirror (a general term for a Z-X bar mirror and Z-Y bar mirror) using focus sensors is excellent in allowing self calibration solely by the apparatus using, e.g., a wafer instead of using special machines. Not only a Z bar mirror but also an X-Y bar mirror is thought to deform due to a temporal change or a shock upon resetting the apparatus (upon zero seek). Since a bar mirror requires periodical shape measurement to avoid such situations, the merit of requiring no special machines in this technique is very important.
FIG. 4 shows a method other than that which uses a bar mirror as a reference of the Z position and tilt. In this technique, a stage which allows driving in the Z and tilt directions is provided on an X-Y stage which slides the X-Y plane using a stage surface plate surface as a reference, and a linear encoder measures the distance between an X-Y stage (stage surface plate surface reference) and the Z/tilt stage. The former and latter techniques are different in whether the measurement target is a bar mirror or a stage surface plate surface, and this means that the same logic applies to both of these measurement targets. Although a method of measuring the Z position/tilt using a bar mirror will be exemplified hereinafter, quite the same applies to a case in which a stage surface plate surface is measured.
As described above, it is possible to precisely measure the shape of a Z bar mirror using focus sensors and a wafer. As a consequence, by correcting an error component in the Z direction in advance before driving a stage in the X and Y directions, the positioning accuracy in the Z direction improves and the focus accuracy of an exposure apparatus, in turn, improves.
Nevertheless, the bar mirror may gradually or abruptly deform due to various factors. For example, when the bar mirror is fixed with a screw, it tends to gradually restore its original shape with time due to a stress by the screw clamp. This exemplifies a case in which the bar mirror gradually deforms over a relatively long period of time. Note also that since a laser interferometer is a relative position measurement system, the origin position of the stage must be determined upon powering on the apparatus. At this time, the origin position of the stage is often determined by mechanical abutting against it. In this case, a butting force acts on the stage, and this may deform the bar mirror depending on circumstances involved.
The same applies to a case in which a bar mirror is fixed in position using an adhesive, i.e., the bar mirror may deform due to a temporal change in the property of the bonding surface or a mechanical butting force against the stage.
In other words, it is very difficult to mechanically inhibit a change in the shape of the bar mirror with an accuracy on the nanometer order. Even if the shape of the Z bar mirror can be corrected with high accuracy by the method described in Japanese Patent Laid-Open No. 2001-015422, the positioning accuracy in the Z direction often degrades gradually or abruptly.
To overcome this problem, it is necessary to periodically measure the shape of the Z bar mirror by the above-mentioned method. Unfortunately, even in this case, the productivity of the exposure apparatus lowers because of the necessity of measurement while the exposure process is stopped. The rate of deformation of the bar mirror varies depending on the performances of individual exposure apparatuses, so it is hard to know an appropriate measurement timing. If the measurement is performed at too long an interval, the amount of deformation of the bar mirror becomes too large, and this may produce defective products. Conversely, if the measurement is performed too frequently, the operating rate of the exposure apparatus lowers, resulting in degradation in the productivity. Furthermore, in both cases, the above-mentioned method cannot cope with a situation in which the bar mirror has deformed abruptly.